Syrian opposition groups last night poured scorn on President Bashar al-Assad's offer of an amnesty to his opponents, with one of the most senior members of the Muslim Brotherhood saying the time had come for foreign military intervention.

Mr Assad, who has faced a 10-month insurrection that shows no sign of fizzling out, tried to stave off the threat to his 11-year rule by offering to pardon those guilty of "crimes" committed since his crackdown. The announcement, which came as observers from the Arab League concluded their visit to the country, was derided by opposition groups.

"Anything he has announced in the past has never been implemented," said Basma Kadmani, a leading member of the main opposition group, the Syrian National Council. "I think our scepticism is justified."

Dr Kadmani said she saw "no chance" that soldiers who had defected would return to the national army following President Assad's offer. Yesterday, one of the most senior members of Syria's Muslim Brotherhood, Molham al-Drobi, explicitly called for foreign military action. "We are only asking the international community to protect our sons and daughters who are being killed in cold blood," he told The Independent.

He demanded an Arab military deployment or United Nations peacekeeping mission and said: "They need to send forces immediately."

Mr Assad's offer of an amnesty was being extended to protesters who had joined demonstrations or handled illegal weapons during the uprising, the state-run Sana news agency said. It also referred to the thousands of soldiers who defected to take up arms with the rebel Free Syrian Army (FSA). A tersely worded statement on the news agency's website added: "The fugitives cannot benefit from the decree unless they turn themselves in prior to 31 January 2012."

Thousands of troops have deserted and now operate under the umbrella of the FSA, a force cobbled together in recent months and whose commanders have taken refuge in mountains in southern Turkey. The FSA claims it is 40,000-strong but other estimates put it at 10,000. Whatever the true figure, the presence of this large-scale guerrilla operation has heightened fears that Syria could descend into full-blown civil war.

Amid mounting pressure on Mr Assad, the ruler of Qatar, Sheikh Hamad bin Khalifa Al Thani, said on Saturday that Arab troops should be dispatched to Syria in a bid to end the bloodshed. "This is a signal that Arab cover for Syria is dissipating," said Andrew Tabler, an expert on Middle East affairs.

The fragmented nature of the opposition was underscored yesterday as the Muslim Brotherhood's call for foreign intervention was rejected by a spokesman for the National Co-ordination Committees. "They do not want the Arab League initiative to be applied here," said Dr Abdel-Aziz al-Khair. "They want people to take up arms against the regime."

Officials of the Assad regime bowed to pressure from the Arab League last month and allowed observers into Syria to check that Damascus was honouring its pledge to halt its crackdown and withdraw the military from urban areas. But, according to the Local Co-ordination Committees, a network of activists inside Syria, the military yesterday aimed shells at the mountain town of Zabadani, in the south-west, just hours before the arrival of a team of international observers.

The United Nations Secretary-General, Ban Ki-moon, used his starkest language yet as he called yesterday for Mr Assad to end his crackdown, which the UN estimates has claimed more than 5,000 civilian lives.

Mr Ban told a UN conference on democracy in the Arab world, meeting in Beirut: "Today I say again to President Assad of Syria: "Stop the violence. Stop killing your people. The path of repression is a dead end. The winds of change will not cease to blow. The flame ignited in Tunisia will not be dimmed."